What you're describing could very well be the result of a virus. Post a HijackThis log on the Malware Removal forum just to make sure.
Go to this site
http://www.trendsecure.com/portal/en-US/threat_analytics/hijackthis.php?page=download.
Click on 'Download HijackThis Installer'.
*Save HJTInstall.exe to your desktop, or to the location where you normally download.
*Doubleclick on HJTInstall.exe.
*Click on Install.
*By default it will install to C:\Program Files\Trend Micro\HijackThis.
*Once installed, it will launch Hijackthis.
*Click on the 'Do a system scan and save a logfile button'. It will scan and the log will open in notepad.
*Click Save to save the log file.
*Click on 'Edit > Select All' then click on 'Edit > Copy' (or Ctrl-C) to copy the entire contents of the log.
*Start a new thread and Paste (or Ctrl-V) the log.
*DO NOT have Hijackthis fix anything yet. Most of what it finds will be harmless or even required.